This application claims the priority of German Application No. 100 60 791.8, filed Dec. 7, 2000, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method and an apparatus for conveying a cryogenically-stored fuel to an internal-combustion engine for providing a combustible work gas.
Low-boiling fuels, such as hydrogen, can be stored in insulated tank arrangements. The latter, also known as cryo-tanks, are distinguished by a relatively low integral weight, as well as a favorable structural space/storage capacity ratio. The tank arrangements provided for storing the cryogenic fuel, that is, fuel that has been liquefied through the extraction of heat, are typically filled over time with fuel that has been sufficiently supercooled. In tank arrangements for storing hydrogen, fuel temperatures below 20 K are necessary. The fuel can be diverted from the tank in the gaseous state, and conveyed into a mixing system, in which the gaseous fuel is mixed with an oxidation agent, such as the oxygen contained in the ambient air, according to a predetermined stoichiometric ratio. It is also possible to extract the fuel from the tank while the fuel is in liquid form. In this connection, German Patent document DE 696 02 468 T2, for example, describes a cryogenic pump that is distinguished by a low NPSH value, and therefore permits the conveyance of the fuel close to the boiling state. The known cryogenic pumps, however, have a complicated design, and exhibit a reduced conveying power if the fuel has been stored for extended periods of time and the tank fullness level is low.
It is the object of the invention to provide a method and an apparatus for conveying a cryogenically-stored fuel to an internal-combustion engine, the method and apparatus reliably ensuring a sufficient fuel flow from the fuel tank.
In accordance with the invention, this object is accomplished by a method for conveying a cryogenically-stored fuel, in the liquid state, to an internal-combustion engine, with a conveying device conveying the cryogenically-stored fuel from a thermally-insulated fuel tank and into a fuel line, and with the pressure in the fuel tank being increased, at least from time to time, during the operation of the conveying device by an amount that is larger than a dominant intake-pressure gradient on the intake side of the conveying device.
Thus, it is advantageously possible to maintain the fuel in a boiling state for natural cooling until immediately before the motor vehicle is started up. It is further possible to extensively empty the fuel container. The reliable elimination of the risk of cavitation allows for the omission of the pre-conveying devices that have been necessary to this point.
The pressure in the fuel tank is advantageously increased to a level that is higher than the sum of the boiling pressure and the intake-side pressure drop at the conveying device.
In accordance with a particularly preferred embodiment of the invention, the pressure in the fuel tank is increased through the supply of gaseous fuel into the fuel tank. The gaseous fuel is preferably drawn from a pressurized tank and conveyed into the fuel tank.
The gaseous fuel conveyed into the fuel tank can advantageously be diverted from a site in the fuel line that is located downstream of the conveying device, in which instance the diverted fuel is heated and evaporated. The heat supplied to the diverted fuel can preferably be drawn from the environment via a heat exchanger embodied as an evaporator.
In accordance with a particularly preferred embodiment of the invention, the conveying device is driven by a motor that is disposed in the fuel tank and cooled by the fuel. The conveying device preferably conveys the fuel at a pressure level in the range of an internal-combustion intake pressure. Consequently, it is possible to form the combustible working gas in the combustion or work chamber of an internal-combustion engine within the framework of an internal mixture-formation process.
Immediately after the conveying device has been shut off, the pressure in the tank is preferably decreased to the boiling pressure dictated by the fuel temperature. Thus, it is possible to cool the fuel through natural evaporation. This natural evaporation is preferably controlled through the continuous control of the tank pressure. The fuel changing into the gaseous state can be supplied to a secondary consumer, particularly a fuel cell, an auxiliary heating device or an [APU]. It is possible to use the released fuel to operate a cooling system, which additionally cools the tank contents.
With respect to the apparatus, the object mentioned at the outset is accomplished in accordance with the invention by an apparatus for conveying a cryogenically-stored fuel, in the liquid state, to an internal-combustion engine, having a thermally-insulated fuel tank for storing the fuel, a conveying device for conveying the fuel into a fuel line, and a booster device for selectively increasing the pressure in the fuel tank by an amount that is larger than the difference between a drop in the supply-flow pressure occurring on the intake side of the conveying device and the difference between the dominant tank pressure at a pump-inlet point and the boiling pressure dictated by the instantaneous fuel temperature.
It is therefore possible to reliably convey the fuel without the risk that cavitation will impede the conveying capability.
The booster device preferably includes a fuel container for storing the fuel, in the gaseous state, at a pressure level that is higher than the tank pressure. A valve device is preferably provided for controlling the flow of the gaseous fuel into the fuel tank. The valve device can be controlled by a control device, so the dominant tank pressure adheres to a defined control concept.
An evaporator device is preferably provided for supplying heat to a partial fuel flow diverted from the fuel line. A check-valve device can advantageously be disposed in front of this evaporator device for preventing a reflux into the fuel line.
The conveying device is preferably hydrostatic. In this case, high conveying pressures can be attained if the conveying device has a comparatively low power capability. Suitable hydrostatic conveying devices include pistons, gear pumps and/or diaphragm pumps. The valve members provided on the intake side are preferably actuated by drive members, so no considerable pressure drop prevails at these valve members.
The conveying device is preferably disposed in the fuel tank, and includes an electric motor that has a virtually superconducting motor winding.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.